Five monoclonal antibodies to the matrix protein of the influenza A virus have defined three antigenic sites. Two of the three undergo genetic variation of a type illustrative of genetic dimorphism also seen with surface antigens. Monoclonal antibodies to the four epitopes of the A/Udorn/72 hemagglutinin were produced that recognized both glycosolated and unglycosolated hemagglutinins produced in tissue cultures infected with a SV40-HA recombinant. IgA monoclonal antibodies have been produced to influenza A virus proteins in an effort to characterize the local antibody response to viral pathogens. Monoclonal antibodies to paramyxovirus and respiratory syncytial virus are being produced to characterize the antigenic composition of the virus proteins.